Semiconductor charge transfer devices generally are of two basic types: charge coupled devices (CCD) and bucket brigade devices (BBD). Either of these types can be used for transfer of charge packets and can be built in the form of a transversal filter device, that is, a tapped delay line configuration with suitably weighted outputs. Such a filter device contains many stages, typically of the order of 10 or more, each stage containing a split-electrode having two electrode segments for sensing the charge packet in that stage. Typically, the lengths (l.sub.1 and l.sub.2) of the two segments of such a split-electrode pair in a given stage are characterized by a ratio, r = l.sub.1, /l.sub.2, in accordance with a predetermined tap weight for that stage; whereas the sum of the lengths (l.sub.1 + l.sub.2) of the two segments of a split-electrode pair is the same for all such split-electrode pairs, corresponding to the width of the charge transfer channel. The effective tap weight r.sub.i of that stage is then given by: r.sub.i = (l.sub.1 - l.sub.2)/(l.sub.1 + l.sub.2). One electrode segment (l.sub.1) of every split-electrode is ohmically connected to a first common output terminal for the charge transfer device, and the other electrode segment (l.sub.2) of every split-electrode is ohmically connected to a second common output terminal for the charge transfer device. For convenience of description, all the electrode segments that are connected to the first common output terminal are referred to as forming the "first set" of sense electrodes, and all the electrode segments that are connected to the second common output terminal are referred to as forming the "second set" of sense electrodes. During operation of such a charge transfer device of the split-electrode type, there will be a sequence of periodic time intervals (or time slots) during which each of these split-electrode segments is sensitive to the corresponding underlying charge packet in the semiconductor by reason of induced electrical image charges, so that signals (S.sub.1 and S.sub.2) are periodically developed at the output terminals respectively of the first and second set of electrodes, each such signal being proportional to the sum of the various charge packets underlying all the various electrodes in that set, with each such packet multiplied by the corresponding tap weight. The desired output signal of the device is then the sequence of instantaneous differences between the signals periodically developed during the aforementioned time slots at the two output terminals; that is, the desired (difference mode) output signal (S.sub.1 -S.sub.2) for a given time slot is proportional to: EQU .SIGMA.Q.sub.i (1 + r.sub.i)/2 - .SIGMA.Q.sub.i (1 - r.sub.i)/2 = .SIGMA.Q.sub.i r.sub.i ( 1)
wherein r.sub.i is the effective tap weight of the split-electrode pair of the i'th stage, and Q.sub.i is the charge packet in the i'th stage during the given time slot.
In order for a transversal filter to function properly, it is important that there be a substantially linear relationship between the input signal and the corresponding output signal. In the semiconductor charge transfer device operating as a transversal filter, it is thus important to have a linear relationship not only between charge packet and corresponding input signal but also between charge packet and corresponding output signal. However, the voltage on a sense electrode in a charge transfer device has an important influence on the width of the depletion layer in the semiconductor underlying the electrode; hence, the voltage on a sense electrode has an important influence on the "image" charge induced on the sense electrode by an underlying charge packet in the semiconductor, because of the tendency of every charge packet to image a portion of its charge into the semiconductor substrate across the depletion layer (rather than into the sense electrode) depending upon the local depletion layer width (and hence upon depletion layer capacitance). Since the voltage on a given sense electrode in one set of sense electrodes depends upon the charge induced by the various charge packets underlying all the other electrodes in that set, the charge packets underlying all these other electrodes undesirably influence the "image" charge induced by any charge packet underlying the given electrode. Thus, the relationship of the charge packet in a given stage to the "image" charge induced on the overlying electrode is distorted from the ideal value, that is, the output signal is not the desired value because the output is not proportional to the display of Eq. (1). This undesirable phenomenon is called "crosstalk" and causes distortion of the output signal.
A measure of the overall distortion in the output signal at a given moment of time caused by the effects of "crosstalk" is the sum total of all the charges instantaneously present in all the charge packets in the transversal filter device. This sum total of all the charges is reflected in the common mode signal (S.sub.1 + S.sub.2) on the two sets of electrodes: EQU .SIGMA.Q.sub.i (1 + r.sub.i)/2 + .SIGMA.Q.sub.i (1 - r.sub.i)/2 = .SIGMA.Q.sub.i ( 2)
This common mode signal ordinarily is large compared with the desired difference mode signal, and thus the detection process is made difficult by reason of the need for detecting the relatively small difference signal (Eq. 1) in the presence of a relatively large common mode signal (Eq. 2). R. D. Baertsch et al., in a paper entitled "The Design and Operation of Practical Charge-Transfer Transversal Filters" published in IEEE Transactions on Electron Devices, Vol. ED-23, No. 2, pp. 133-141 (February, 1976), have disclosed detection circuits for a CCD transversal filter. However, in all those circuits the difference mode signal is detected by an amplifier which must also handle the common mode signal, thereby necessitating costly and cumbersome circuit elements and configurations. Accordingly, it is desirable to suppress the distortion caused by the common mode signal in a semiconductor transversal filter device by less costly means than in the prior art.